I only started singing in public a few months ago and at a time in my life when remembering words is probably not getting easier but this is what I've found so far -
Songs that repeat and have a simple story are easiest - e.g. Prickle-eye Bush
Also songs you have known and sung along to over the years
Sing it as often as you can start to finish, around the house, at the bus-stop etc
DO NOT GET DISTRACTED - as soon as my mind takes in anything else, however insignificant, the words disappear.
If it helps when performing, close your eyes
When singing the chorus, start thinking about the next verse
If you get your lines or words round the wrong way plug on and ignore it, don't stop or apologise
If you find words that have much the same meaning but suit the way you sing the song better than the originals use those instead
And if you really want to do well at a particular venue then sing it in public somewhere else first - I find I am much better at remembering the words after a performance than before or during it.